Familiar to every desert explorer, certain dark shiny walls stand out from the normally red cliffs of mesas and canyons in the southwestern United States. Native Americans for centuries etched drawings into them. The ubiquitous desert varnish that coats sunlit walls of sandstone in Utah and other desert environments around the world, though, has long been an enigma to scientists. How does it form? Why does it form? Now, they believe they have the answer: the artists are photosynthetic bacteria.
In their commentary in PNAS, “Shining light on photosynthetic microbes and manganese-enriched rock varnish,” Valeria C. Culotta and Asia S. Wildeman are glad that a comprehensive explanation has finally arrived.
The varnish is a metal coating several hundred microns deep, composed largely of black oxides of manganeseor orange-colored iron oxides. Many ancient petroglyphs of Native Americans were created by etching into the black or orange coat of varnish, exposing the lighter rock underneath. The blackened manganese-rich varnish contains Mn3+ or Mn4+ oxides at concentrations two to three orders of magnitude higher than manganese levels in neighboring soils or rock. These varnishes develop very slowly over time, requiring thousands of years in the making. In a paper by Lingappa et al. the secrets of rock varnish genesis are unveiled by the discovery of microbes that inhabit desert rock and deposit manganese footprints as their legacy. [Emphasis added.]
Desert varnish, it turns out, is a product of generations of microbes stacking their work, like stromatolites, on top of the work of previous generations. Varnish increases in thickness over time. But it doesn’t take inordinately long, either, because new varnish can be found on top of earlier etchings. The bacteria utilize doubly ionized manganese (Mn2+) while alive. After they die, other bacteria or abiotic processes convert that to Mn3+ or Mn4+ oxides. This explains the earlier observations.
Manganese (atomic number 25, molecular weight 55) is the 12th most abundant element on earth, and the 5th must abundant metal; even so, that amounts to only 0.1 percent of earth’s crust. It is far less abundant in the solar system at large (360 ppm on Earth, 0.2 ppm elsewhere). According to the Chemicool website that documents such things, manganese was identified as a separate element in 1740. The Romans had used black manganese dioxide to make colorless glass — a function still in practice today. It’s used primarily in alloys now and finds its way into plastic bottles and aluminum cans, giving them stiffness. In life, it is an essential trace element for photosynthesis. All living things, including humans, need to ingest manganese:
In the human body several manganese-containing enzymes are need[ed] to metabolize carbohydrates, cholesterol, and amino acids. Typically our bodies have about 10 – 20 mg manganese. This needs to be topped up frequently because our bodies cannot store it. About a quarter of the manganese in our bodies is in bone, while the rest is evenly distributed through our tissues.
Michael Denton includes a section on manganese in his short book The Miracle of the Cell, where he describes the element’s crucial role in chloroplasts for oxidizing water. The compound Mn4Ca resides at the heart of an enzyme that enables plants to release oxygen to the atmosphere and use the spare electrons for the manufacture of biomolecules. Without that compound, life as we know it would be rare to nonexistent. Last year, Evolution News discussed how manganese and other trace metals are delivered in usable form to the surface of the earth by glaciers — one of several geological processes that ensure that essential elements are available for living organisms.
Microbes Involved in Desert Varnish
Having five free electrons, manganese can take on multiple redox states in various oxides which, like iron oxides, can be colorful. The manganese oxides on desert walls take on a shiny dark purple color. Scientists had discovered microbes in the varnish but they lacked an explanation for how the microbes deposited manganese on the rock in such high concentrations. Culotta and Wildeman explain the breakthrough made by the Lingappa team:
What was missing from these initial analyses of varnish microbes was an explanation for the origin of manganese. Regardless of its redox state, how does manganese in such abundance get there in the first place? A breakthrough was obtained by Lingappa et al. through analyses of the physical, microbiological, and bioinorganic properties of diverse samples of desert varnish. A principal finding was the strong enrichment of the Xenococcaceae family of Cynanobacteria at all sites of desert varnish examined. The evidence shows these photosynthetic bacteria are not a mere passenger but rather a driver of desert varnish, responsible for depositing manganese in high concentrations at varnish locales.
In short, desert varnish is a product of photosynthetic cyanobacteria! On those dry desert cliffs, living cells are growing and releasing oxygen to breathe. This explains why the material is favored on sunlit walls. Not only that, the bacteria also convert the much-discussed greenhouse gas carbon dioxide into forms of carbon that neighboring cells can use for their Calvin cycle, “thereby promoting microbial growth in the otherwise nutrient-sparse environment of the desert.” The astonishing result is an “invisible intricate ecosystem that over thousands of years created [these] beautiful glittering rocks.” Climatologists will undoubtedly be glad for the natural carbon-capture process, too.
How do microbes concentrate manganese at levels hundreds or thousands of times higher than the surrounding environment? Apparently, the answer is blowing in the wind. Dust carried by wind delivers this trace element to the cyanobacteria, which are able to import it and put it to use. Surprisingly, the Mn is “not bound to proteins or other macromolecules but rather to small organic or inorganic compounds.”
But questions remained; why do these microbes need so much manganese? Cyanobacteria need about 100,000 atoms of Mn per cell to run photosynthesis, but these species were found to contain 100 million atoms per cell. Why? Chroococcidiopsis, the most abundant species on rock walls, uses the manganese clusters for sunscreen!
Although the nature of the Mn2+ complex(es) is still unknown, its redox activity is likely to bestow Chroococcidiopsis with extreme resistance to the damaging ROS [reactive oxygen species] effects of excessive desiccation, heat, and ionizing radiation.
A Bio-Geosphere Modulated by Microbes
What a remarkable thing to learn: what were once considered “primitive” cells are creating vast murals of living art on cliff walls around the world — not just for show, but for oxygen effusion and carbon capture, processes that benefit the entire biosphere. And they do this in some of the driest, hottest, and exposed habitats on earth!
The role of microbes involved in massive geophysical processes is becoming more evident. Bacteria are found in biocrusts, helping higher organisms establish a foothold in sandy deserts. They are implicated the formation of cave speleothems. And now we see them setting up shop on desert cliffs, creating ecological murals showing off biochemical wizardry. Bacteria travel the world in winds and clouds, bringing their expertise to some of the most inhospitable parts of the planet. Unlike the abiotic dust that carries them, microbes all operate on encoded information that directs molecular machines to perform work that is both functional and beautiful. In the original paper in PNAS, Lingappa et al. say,
The understanding that varnish is the residue of life using manganese to thrive in the desert illustrates that, even in extremely stark environments, the imprint of life is omnipresent on the landscape.
Rock Art Palimpsests
There’s an enthusiastic subculture of desert hikers and off-roaders — scientists, too — who understandably delight in finding rock art on canyon walls. It’s like a treasure hunt. They go to great lengths sometimes to find these silent messages left by former human societies who chipped figures of themselves and their animals and deities into the desert varnish down to the sandstone underneath. The artwork reveals something beyond nature: something about the minds of intelligent agents who took the time to leave their marks for posterity. If the hobbyists knew that the artwork in the chipped-away material was even more fascinating, what would be the reaction? Would it be like peeling away scratch paper containing stick figures to discover a masterwork underneath?
The tools of science are permitting us to peel away the surfaces of commonly encountered environments to reveal palimpsests of design that were always there but never seen before. Some of them are real wall-hangers.